1. Field of the Invention
The present invention relates to a superconducting device, more particularly a super-FET having a novel structure having a flat surface and a weak link which is formed locally at a portion of a thin film of oxide superconductor deposited on a substrate, and a method for manufacturing the same.
2. Description of the Related Arts
The conventional superconductors exhibit the superconductive phenomenon only at limited extremely low temperatures so that it has not expected to be used in actual applications. However, new type oxide superconductors of La, Ba!.sub.2 CuO.sub.4, La,Sr!.sub.2 CuO.sub.4 or the like were discovered in 1986 and then discovery of the other superconducting compound oxides such as Y-Ba-Cu-O system or Bi-Ca-Sr-Cu system was continued. In these newly discovered oxide superconductors, the superconductive phenomenon can be realized with relatively cheaper liquid nitrogen and hence the possibility of actual utilization of the high Tc superconductors have burst onto the scene.
The oxide superconductors were used in a bulk form of sintered block by powder sintering technique. The sintered articles, however, shows very poor critical current density. In order to overcome this problem, a variety of deposition techniques have been studied to prepare thin films of these superconductors.
Josephson device which is a typical superconducting device has a pair of superconducting electrodes connected through a so-called weak link and shows unique properties such as DC Josephson effect which is explained as a tunnel effect of Cooper pairs and AC Josephson effect which is characterized by dispersive voltage/current curve. A planer type Josephson device which can be manufactured from a thin film of oxide superconductor can be classified into two categories of a Dayem bridge (DMB) type and a variable thickness bridge (VTB) type. FIG. 4A and 4B illustrate typical two structures of these two types.
FIG. 4A shows a simplest form of the Dayem bridge (DMB) type Josephson device. This Josephson device has a pair of superconducting electrode zones 21b, 21c each having a sufficient line width which are produced by patterning work on a thin film 21 of oxide superconductor having a sufficiently thick wall-thickness and deposited on a substrate 11 and a weak link zone 21a having an extremely fine line width (w).
FIG. 4B shows a simplest form of the variable thickness bridge (VTB) type Josephson device2 This Josephson device has a pair of superconducting electrode zones 21b, 21c each having a sufficient line thickness which are produced by removing a predetermined portion of a thin film 21 of oxide superconductor and deposited on a substrate 11 and a weak link zone 21a having an extremely thin wall-thickness (t).
In these planer type Josephson devices, the line width (w) or the wall-thickness (t) of the weak link has a close relation to performance of the superconducting devices obtained and hence, in order to obtain a desired characteristics or performance with a good repeatability, a high precision on a sub-micron level of the processing such as the etching is required. In fact, the planer type Josephson devices have following problems:
The Dayem bridge type Josephson device can be said to be more preferable than the variable thickness bridge type Josephson device, since the Dayem bridge type Josephson device has a relatively planer surface, which is preferred in integrated circuits. However, in order to form the weak link in the Dayera bridge type Josephson device, it is required to pattern a thin film of oxide superconductor having a thickness on the order of 0.5 .mu.m to 1.0 .mu.m into a width of not greater than 0.2 .mu.m. However, it is very difficult to conduct such fine patterning with good repeatability. On the other hand, in the variable thickness bridge type Josephson device, the very fine pattering is not required in order to form the weak link. However, it is very difficult to uniformly control the remaining thickness of the thinned portion forming the weak link. In addition, the variable thickness bridge type Josephson device cannot have a satisfactorily planer surface. This is not preferable to the integrated circuit applications.
Superconducting devices having three-terminals such as a superconducting-base transistor, a super-FET (field effect transistor) or the like are also known.
FIG. 5 is an illustrative drawing of a superconducting-base transistor having a layered structure comprising an emitter 121 made of superconductor or ordinary conductor, a tunnel barrier 122 made of insulator, a base 123 made of superconductor, an isolator 124 of semiconductor and a collector 125 made of ordinary conductor arranged in this order. The superconducting-base transistor is a high-speed element of low power consumption in which high-speed electrons pass through the tunnel barrier 122.
FIG. 6 is an illustrative drawing of a super-FET in which a superconducting source electrode 141 and a superconducting drain electrode 142 each made of superconductor are arranged on a semiconductor layer 143 in closed proximity to each other. A portion of the semiconductor layer 143 between the source electrode 141 and the drain electrode 142 is removed at its rear side to form a thin layer on which a gate electrode 144 is deposited through a gate insulator layer 146. In the super-FET, a superconducting current flows through the semiconductor layer portion between the superconductor source electrode 141 and the superconductor drain electrode 142 due to a superconducting proximity effect, and is controlled by an applied gate voltage. This super-FET also functions at a high speed with a low power consumption.
The other type three-terminal superconducting devices are also reported. For example, a known three-terminal superconducting device has a channel of a superconductor formed between a source electrode and a drain electrode so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel.
The above mentioned superconducting-base transistor and the super-FET have a portion in which a semiconductor layer and a superconducting layer are stacked one over another. However, it is difficult to form such a stacked structure of the semiconductor layer and the superconducting layer formed of an oxide superconductor which has been recently advanced in study. And, even if it is possible to form such a stacked structure of the semiconductor layer and the oxide superconducting layer, it is difficult to prepare a desired interface or boundary between the semiconductor layer and the oxide superconducting layer. Therefore, a satisfactory operation and performance could not been observed in the conventional superconducting devices.
Still more, since the super-FET utilizes the superconducting proximity effect, the superconductor source electrode 141 and the superconductor drain electrode 142 must be located closely to each other at a close distance which is a few times of the coherence length of the superconductor materials of the superconductor source electrode 141 and the superconductor drain electrode 142. In particular, when the superconductor source electrode 141 and the superconductor drain electrode 142 are formed of the oxide superconductor material, a distance between the superconductor source electrode and the superconductor drain electrode has to be not greater than a few ten nanometers because the oxide superconductor has a very short coherence length. However, it is very difficult to conduct a fine processing such as a fine pattern-etching so as to ensure such very short separation distance. Because of this, in the prior art, it has been impossible to manufacture the super-FET composed of the oxide superconductor material.
Although some modulation operation or behavior was confirmed in the conventional three-terminal superconducting device having the superconducting channel, the conventional three-terminal superconducting device having the superconducting channel could not realize a complete ON/OFF operation because of its too high carrier density. The oxide superconductor is a material expected to realize a new three-terminal superconducting device which has a superconducting channel and which can realize a complete ON/OFF operation since the oxide superconductor material has a low carrier density. In this case, however, a thickness of the superconducting channel has to be made on the order of five nanometers.
Therefore, an object of the present invention is to solve the problems of the prior arts and to provide a novel structure of the superconducting device whose upper surface can be flattened easily and which can be manufactured by using existing established processing techniques with a good repeatability and a method for manufacturing the same.